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Unusual Change in Critical Frequency of F<sub>2</sub>Layer during and Prior to Earthquakes

Open Journal of Earthquake Research 2017年第4期6卷 191-203页

作者： Soujan Ghosh sudipta sasmal Subrata Kumar Midya Sandip K. Chakrabarti Indian Centre for Space Physics Kolkata India Department of Atmospheric Sciences Calcutta University Kolkata India S. N. Bose National Centre for Basic Sciences Kolkata India

The unusualness in the critical frequency of different layers of earth’s ionosphere is commensurate to be associated with seismic events. We present study of critical frequency of F2 layer (denoted as f0F2) during some major earthquakes in South American region. We use the semi-empirical Barbier’s theorem of air-glow and define a parameter using the critical frequency and virtual height of F2 layer and named it as “F Parameter”. To investigate the variation of this parameter, we consider five large earthquakes in the junction of Nazca plate and South American plate having magnitude greater than M > 6.5 and study the temporal variation of F parameter during these earthquakes. The F Parameter is measured using the ionograms as recorded from the Ionosonde in Jicamarca Radio Observatory (lat. 11.95°S, long 76.87°W) in Chile which lies within the earthquake preparation zones of these five earthquakes. We examine the F Parameter within a span of ±15 days during earthquakes and observed significant change in the evaluated F Parameter in 12 to 3 days prior to the earthquakes. The increment is over +3σ from the normal variation. We also observe significant changes during aftershock events. The solar geomagnetic indices were found to be low which ensures that these anomalies in F Parameter are due to seismic events.

关键词： Earthquake Early Warning Wave Propagation Earthquake Interaction Forecasting and Prediction Ionosphere/Atmosphere Interactions Electrical Properties

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Contaminated Effect of Geomagnetic Storms on Pre-Seismic Atmospheric and Ionospheric Anomalies during Imphal Earthquake

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Open Journal of Earthquake Research 2020年第5期9卷 383-402页

作者： Sagardweep Biswas Subrata Kundu Soujan Ghosh Swati Chowdhury Shih-Sian Yang Masashi Hayakawa Suman Chakraborty Sandip K. Chakrabarti sudipta sasmal Indian Centre for Space Physics Kolkata India Institute of Space Science National Central University Chinese Taipei Hayakawa Institute of Seismo Electromagnetics Co. Ltd. UEC Alliance Center Chofu Tokyo Japan Physical Research Laboratory Navrangpura Ahmedabad India

Study of seismo-ionospheric coupling mechanism demands the quiet geomagnetic condition to eliminate any kind of contamination in the lower atmospheric and ionospheric parameters. In this manuscript, we present the effect of back to back two geomagnetic storms before a strong earthquake happened in Imphal, India on January 4, 2016 (M = 6.7). We studied the lower ionospheric irregularities for the duration of 31 days by computing the nighttime fluctuations in Very Low Frequency (VLF) radio signal received transmitter JJI (22.2 kHz) in Japan at Ionospheric and Earthquake Research Centre & Optical Observatory (IERCOO), Sitapur, India. We also studied the presence of Atmospheric Gravity Wave (AGW) in nighttime VLF signal in lower ionospheric heights and the same computed that from SABER/TIMED satellite. Two geomagnetic storms occurred on December 21, and 31, 2015. By the conventional analysis, we found that there is a significant decrease in nighttime trend and an increase in nighttime fluctuations around 15 days before the earthquake and just on the first storm and thus the pre-seismic effects on VLF signal gets contaminated due to the presence of storms. The wave-like structure in VLF fluctuations shows significant increase in intensity by using Fourier and Wavelet analysis before the earthquake. By analysis of SABER data, we found significant enhancement in AGW around 10 days before the earthquake. As the wavelike structures are coming from neutral acoustics reasons from pressure or temperature variations, this paper exhibits a significant example of contamination in ionospheric parameters due to geomagnetic storm where the acoustics parameters remain un-contaminated.

关键词： Very Low Frequency Wave Propagation Earthquake Precursory Atmospheric Gravity Wave Geomagnetic Strom SABER/TIMED

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